Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin

Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND NO. 2011-XXXXP

Protection of Renewable-rich

Microgrid

Mohamed E. El Khatib

Sandia National Laboratories

Outline

Microgrid Protection Challenges

Current Protection Practices

Advanced Protection Schemes

Impedance-based Protection

Transient-based Protection

Microgrid Protection Challenges

Variety of sizes, technologies, configurations radial, meshed, dynamic topology

difficult to have a “one size fits all” solution.

Islanded and grid-connected modes of operation Significantly different fault levels makes coordination challenging.

Fault levels could be very sensitive to generation dispatch thus complicating coordination.

Inverter-rich Microgrid could have too low fault current Overcurrent might not detect the fault in the first place.

High-impedance faults are particularly problematic.

Efficient microgrid protection schemes will also be beneficial for protecting distribution systems with very high penetration of

renewable generators.

Microgrid Protection Needs

• Development of efficient protection schemes for Microgrid is crucially needed to:

• Enable integration of higher penetration of renewables

• Enable the safe deployment and support of “networked Microgrid” for resilience improvement

• Advanced protection schemes are needed to protect against high-impedance faults, which is very hard today

• Current protection schemes are problematic with Microgrid

• Overcurrent protection can’t detect faults with high use of inverters

• Differential protection is expensive to use in many Microgrid applications

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Microgrid Protection Gap

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Microgrid Protection Zones

A protection zone is defined as: “a part of the microgridbordered by a set of fault interrupting devices”

Allows efficient protection for advanced and dynamic boundary Microgrid.

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Impedance-based Protection

• Impedance is a better fault indication than current magnitude especially in low-fault, renewable-rich Microgrid.

• More robust than overcurrent protection and less expensive than differential protection

• However, traditional distance-based impedance protection will be complicated to use for a Microgrid due to feeder tapping, small feeders lengths, etc.

• We have formulated and tested different impedance-based protection designs.

• Impedance-based pilot scheme: impedance elements detect the fault and direction elements locate the fault.

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Impedance Protection Example

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Before the fault….

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After the fault….

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Adaptive Overcurrent Protection

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Adaptive Impedance Protection

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Transient-based Protection

Based on detecting faults by analyzing fault-generated transients.

Implementation in the field faces several current challenges. However, this approach could potentially be less expensive than

differential protection and more robust than overcurrent protection

This approach is promising for both low-fault Microgrids and can be extended to protect against high-impedance faults in both Microgrids and distribution systems.

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Transient-based Protection

Discrimination between fault transients and other non-fault transients could be challenging.

Combination of transient-based protection with voltage-based and zero-sequence protection could potentially provide a robust protection solution.

Communication-assisted transient-based protection could be achieved by comparing transient signatures from different locations to locate the fault, potentially, using PMUs.

Processing time must be kept low to ensure fast fault clearance

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Conclusions

Thorough studies and testing are needed to transition new protection schemes to the field. performance of directional and impedance elements under high

penetration of DERs.

interactions between inverter-dominated Microgrids protection and the hosting DS protection

Integration of adaptive protection functions within Microgrid controllers.

There is a need to develop Microgrid protection design guidelines.

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